This invention relates to the protection of intrinsically safe circuits, which are potentially incendiary. It is particularly concerned with the protection of intrinsically safe circuits which operate within hazardous areas, for example in the presence of flammable gases.
In systems where power supplies feed a number of functional modules, where there may be the presence of flammable gases, there is a need to ensure that the system is safe. This means that the wiring between the power supplies and the modules must be protected in some way so that it is not incendiary, even if the modules are unplugged with the system running. One way to achieve this is to provide electronic power limiters in the output from each of the power supplies. These power limiters define the maximum output voltage and limit the short-circuit current. However, these power limiters are quite complex because they must meet the conflicting requirements of being precise yet operate fast. This complexity has to be duplicated to meet the requirements of other, more exacting systems.
It is an object of the present invention to provide a means of protecting potentially incendiary, intrinsically safe circuits using a simpler system than the relatively complex, conventional power limiters.
It is a further object of the invention to provide means whereby one affords complete protection to the wiring and to any plugs and sockets along the way. Desirably, provided that the inductance and capacitance limits are not exceeded, the wiring from the power supply to the modules is protected from open-circuit and short-circuit faults, so that no special construction is required.
In equipment or assemblies in which the wiring is well-controlled, the segregation between conductors can be controlled in the construction of the circuitry. If this segregation is made infallible, then short-circuit faults need not be considered and one only needs to protect against series breaks. For this, simpler protection schemes are adequate.
If one is concerned only with series breaks, then this removes the constraint to put the electronic protection upstream from the wiring to be protected. It simply has to be somewhere in series with the intrinsically safe circuit. The importance of this to achieving protection in accordance with the invention will be apparent from the description given hereinafter.
In accordance with the invention there is provided a method of protecting intrinsically safe circuits in which a voltage is supplied via a supply circuit to a load, which comprises sensing the voltage at the load and, in the event that a decrease in the said voltage is detected, disconnecting the load in such manner as to prevent any series break in the supply circuit from becoming incendiary.
Also in accordance with the invention there is provided apparatus for protecting an intrinsically safe circuit which includes a load and which is arranged to be supplied via a supply circuit with a voltage from power supply means, the apparatus comprising sensing means arranged to detect the said voltage, and switch means arranged, in response to the detection by the sensing means of a decrease in the said voltage, to disconnect the load in such manner as to prevent any series break in the supply circuit from becoming incendiary.
Also in accordance with the invention there is provided a power system for an intrinsically safe circuit, comprising power supply means, power distribution means connected to the power supply means, and at least one module connected to the power distribution means, the or each module comprising an intrinsically safe circuit including a load, sensing means to detect the voltage supplied to the load, and switch means arranged, in response to the detection by the sensing means of a decrease in the supplied voltage, to disconnect the load in such manner as to prevent any series break upstream from the switch means from becoming incendiary.